The transition from summer to winter, known as fall or autumn, is marked by cooling temperatures and shifting daylight hours. While the season is recognized by observable changes in nature, there is no single, universally accepted date for when it officially begins. The start date depends on which system of measurement is used, offering two distinct definitions: one based on celestial movements and the other on consistent calendar dates.
The Astronomical Start of Fall
The astronomical definition of fall is governed by the Earth’s orbit and axial tilt, marking the precise moment of the Autumnal Equinox. An equinox is when the sun appears directly above the Earth’s equator, resulting in an almost equal duration of daylight and darkness globally. In the Northern Hemisphere, this event marks the beginning of autumn and usually occurs on September 22nd or 23rd.
The specific date varies annually because the Earth’s orbit takes approximately 365.24 days, slightly longer than the 365 days in the standard calendar year. This difference is corrected by the inclusion of a leap year every four years, which causes the exact time of the equinox to shift slightly.
From the moment of the autumnal equinox until the winter solstice, the Northern Hemisphere continues to tilt away from the sun. This gradual change causes day lengths to become noticeably shorter and night lengths to increase steadily through the season. The astronomical definition is scientifically precise, based on the physics of the solar system, but its variable date makes it less predictable for planning or long-term climate analysis.
The Meteorological Start of Fall
In contrast to the astronomical system, the meteorological definition of fall uses fixed dates for the consistent collection and comparison of climate data. Meteorological autumn in the Northern Hemisphere begins on September 1st and concludes on November 30th. This system divides the year into four seasons, each consisting of three full months, aligning neatly with the civil calendar.
Meteorologists and climatologists use this fixed-date system to simplify the calculation of seasonal statistics, such as temperature and precipitation averages. By using constant start and end dates, they can easily compare the weather of one fall season to another. This method provides a consistent framework for analyzing long-term weather patterns and climate trends.
Key Natural Signs of Autumn
Regardless of the official calendar start date, nature’s own progression into autumn is triggered by decreasing daylight hours, a phenomenon known as photoperiodism. The shortening length of the day signals to deciduous trees that the growing season is ending, initiating a process called senescence.
The most recognizable sign of this shift is the change in foliage color, which occurs as trees stop producing the green pigment chlorophyll. As chlorophyll breaks down, the previously masked yellow and orange pigments, known as carotenoids, become visible. Red and purple hues come from anthocyanins, which are newly produced from trapped sugars as the tree prepares to shed its foliage.
This biological preparation for winter is also observable in animal migration patterns and the decline in average daily temperatures. These natural markers often begin weeks before the astronomical equinox, with environmental changes more closely tracking the fixed September 1st start date, especially in higher latitudes.